Most virulent bacteria have carbohydrates on their surface, such as lipopolysaccharides and capsular polysaccharides. Antibodies directed against capsular polysaccharides provide, among other things, enhanced phagocytosis and killing of bacterial cells. Usually there are a number of serotypes of a given bacterial species, for example there are more than 80 known serotypes of Streptococcus pneumoniae related to their carbohydrate capsular structures.
Bacterial polysaccharides are classical examples of antigens that are not T helper cell-dependent, and hence, if they are immunogenic at all, they mainly induce IgM class of antibodies. This is so, because only B cells respond to them, and B cells cannot mediate the memory function as opposed to the T cells, which also mediate immunological booster effects.
In immunologically immature small children, elderly and immunosuppressed persons polysaccharides are known to be poor immunogens or not at all immunogenic.
Therefore, polysaccharide antigens which are chemically conjugated to carriers comprising T cell epitopes are effective as vaccines also for the above mentioned immunologically immature children and immunosuppressed adults.
The vaccine-producing industry has long been searching for a general method of producing conjugate-type vaccines. A general and simple method to produce such vaccines would not only be more practical but would also make process and quality control easier.